The present invention relates generally to a connector and, more specifically, to an optical coupler for single optical fibers.
The employment of fiber optic cables or light guides, also sometimes referred to as optical communication fibers, for the transmission of information-bearing light signals, is now an established art. Much development work has been devoted to the provision of practical low-loss glass materials and production techniques for producing glass fiber calbes with protective outer claddings or jackets. The jackets make them resemble ordinary metalliccore electrical cable upon superficial external inspection. Obviously, if fiber optic cables are to be used in practical signal transmission and processing systems, practical connectors for the connection and disconnection of fiber optic cables must be provided.
Some references will now be given for background in the state of fiber optic art in general. An article entitled, "Fiber Optics," by Narinder S. Kapany, published in Scientific American, Vol. 203, pgs, 72-81, November 1960, provides a useful background in respect to some theoretical and practical aspects of fiber optic transmission.
Of considerable relevance to the problem of developing practical fiber optic connectors, is the question of transfer efficiency at the connector. Various factors, including separation at the point of abutment, and lateral separation or axial misalignment, are among the factors effecting the light transfer efficiency at a connector. In this connection, attention is directed to the Bell System Technical Journal, Vol. 50, No. 10, December 1971, specifically to an article by D. L. Bisbee, entitled, "Measurement of Loss Due to Offset, and End Separations of Optical Fibers." Another Bell System Technical Journal article of interest appeared in Vol. 52, No. 8, October 1973, and was entitled, "Effect of Misalignments on Coupling Efficiency on Single-Mode Optical Fiber But Joints," by J. S. Cook. W. L. Mammel and R. J. Grow.
Fiber optic bundles are normally utilized for only short transmission distances in fiber optic communications networks. On the other hand, fibers are used individually as optical data channels to allow transmission over many kilometers. At present, most fiber optic cables are multi-fiber bundles due to the less stringent splicing requirements, greater inherent redundancy, and higher signal-to-noise ratio. The difficulty in achieving connections between single fibers which are insensitive to axial misalignment problems has created an obstacle to the use of long run single data transmission systems.
Therefore, a connector or coupler is required to eliminate lateral tolerances if low-loss connections are to be obtained in the use of single fiber optical transmission arrangements. "V" groove and metal sleeve arrangements have been used to interconnect single fibers. Reference is made to U.S. Pat. No. 3,768,146 which discloses a metal sleeve interconnection for single fibers.
Another known device, as shown in U.S. Pat. No. 3,734,594, utilizes a deformable annular core having pressure plates at the ends. The fiber ends are inserted into the core and an axial force is applied to the plates to deform the core radially, thereby aligning and securing the fibers.
These prior devices, however, do not readily provide sufficient accuracy for joining and aligning small diameter cores of optical fibers.
Copending application of Charles K. Kao entitled, "Precision Optical Fiber Connector," Ser. No. 613,390, filed Sept. 15, 1975, assigned to the assignee of the present application, discloses a single optical fiber connector in which the ends of mating fibers are precisely aligned and coupled together in the interstice between three like contacting cylindrical rods. The rods are mounted along and around the fibers within an adjustable connector assembly. Means is provided for expanding the interstice to insert the fiber ends and for clamping the rods in position around the fibers. Copending application of Charles K. Kao entitled, "Precision Surface Optical Fiber," Ser. No. 629,210, filed Nov. 5, 1975, assigned to the assignee of the present application, discloses an optical fiber in which the plastic cladding thereof is formed with three rounded indentations along its surface and a thin metal ferrule is formed around the cladding at the mating end of the fiber. A pair of such fibers may be aligned in a three rod arrangements of the type mentioned above.
A heremaphroditic connector for coupling a pair of single optical fibers is disclosed in copending application of Ronald L. McCartney entitled, "Single Optical Fiber Connector," Ser. No. 629,004, filed Nov. 5, 1975, also assigned to the assignee of the present application. The connector comprises a pair of connector members each containing at least one single optical fiber terminated by a termination pin. The pin includes a metal eyelet crimped about the optical fiber in three places providing three, spaced, curved indentations which centrally position the fiber in the pin. When the connector members are mated, the mating termination pins are positioned so that indentations therein are generally aligned. Three arcuate cam or spring members are forced into the indentations in the mating termination pins to accurately laterally align the pins and, hence, the optical fibers therein.
In our copending application entitled, "Single Optical Fiber Connector," Ser. No. 680,171, filed concurrently by herewith, there is disclosed a single optical fiber connector comprising a base plate having a V-groove in its upper surface having a transverse cross-section of an equilateral triangle. Two sets of three equal diameter cylindrical rods lay in the groove, each defining an interstitial space therebetween which receives an optical fiber. The sets of rods have mating end faces which abut each other in the groove. A compression plate is mounted over the base plate to arrange the rods in the V-groove so that the centers of the rods are disposed at the vertices of the same equilateral triangle whereby the fibers in the interstitial spaces between the rods become precisely laterally aligned. Such connector arrangement is particularly suited for a flat cable having single optical fibers.
The purpose of the presnt invention is to provide a new separable single fiber connector based upon the three rod coupling approach disclosed in the aforementioned copending applications, and which will provide a controlled, accurate alignment of mating optical fibers in a manner which will minimize light transmission losses, which is easily manufactured and assembled, and practical for commercial field use.